1. Field of the Invention
The present invention relates in general to devices for measuring a distance and more particularly, to distance measuring devices of laser beam using type which, for measuring the distance between it and an object, transmits laser beam toward the object and detects the laser beam reflected by the object. In fact, the period required for the laser beam to make the round trip is detected. Distance to the object is equal to the one-half of the time elapsed multiplied by the velocity of the laser beam.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional distance measuring device of the above-mentioned type will be described with reference to FIG. 5 of the accompanying drawings.
The conventional distance measuring device 100 shown in FIG. 5 comprises generally a transmitter optical section, a scanning optical section and a receiver optical section.
As shown, the transmitter optical section comprises a laser diode 102, a collimating lens 104, anamorphic prism pair 106 and a beam expansion telescope 108. The highly divergent elliptical output beam from the diode 102 is collimated by the collimating lens 104. The beam is then made circular by the anamorphic prism pair 106 and expanded in the beam expansion telescope 108 to create an output beam that increases in diameter.
The scanning optical section comprises a four-sided polygon mirror 110 and a nodding mirror 112 which together scan a raster from left to right, top to bottom. The polygon mirror 110 rotates about its axis 110a at a given speed, and the nodding mirror 112 swings about its axis 112a synchronously with the polygon mirror 110. Denoted by numeral 114 is a motor for swinging the nodding mirror 112. That is, receiving the transmitter laser beam "Lt" from the beam expansion telescope 108, the polygon mirror produces a number of scan lines per second. These scan lines are framed by the nodding mirror 112. Due to the synchronous motion of the polygon mirror 110 and the nodding mirror 112, the transmitter laser beam "Lt" projected from the nodding mirror 110 scans an object (not shown) from left to right, top to bottom or vice versa.
The receiver optical section comprises generally folding and alignment mirrors 116, a narrowband optical filter 118, a reduction telescope 120 and a laser beam receiver 122 (viz., avalanche photodiode). Due to the synchronous motions of the nodding mirror 112 and the polygon mirror 110, the receiver laser beam "Lr" reflected by the object is directed toward to an inlet part of the folding and alignment mirrors 116 and led to the laser beam receiver 122 through the narrowband optical filter 118 and the reduction telescope 120. Although not shown in the drawing, a control unit is employed which has means for detecting the period required for the laser beam to make the round trip.
However, due to its inherent construction, the above-mentioned conventional device 100 has a weak point in obtaining a sufficiently strong receiver laser beam "Lr". Furthermore, usage of the nodding mirror 112 causes a bulky and heavy construction of the device 100.